Researchers at Georgia Tech have developed an inexpensive and simple prototype of a triboelectric nanogenerator that could be used to produce energy from ocean waves by making use of contact electrification between a patterned plastic nanoarray and water. A report on their work is published in the journal Angewandte Chemie.
Contact electrification, also called triboelectrification, is an old but well-known phenomenon in which surface charge transfer occurs when two materials are brought into contact. Although some of the fundamental mechanisms about triboelectrification are still under discussion, such as what subjects (electrons, ions, or small amounts of material) are transferred during the contact and separation process to produce the charged surface, and why surface charge transfer occurs even between identical materials, triboelectrification does exist and it has some practical applications together with many negative consequences. Recently, contact electrification has been demonstrated for some potential applications, such as energy harvesting, chemical sensors, electrostatic charge patterning, metal-ion reduction, and laser printing.
The triboelectric nanogenerator (TENG), which is the first invention utilizing contact electrification to efficiently convert mechanical energy into electricity, has been systematically studied to instantaneously drive hundreds of light-emitting diodes (LEDs) and charge a lithium-ion battery for powering a wireless sensor and a commercial cell phone. Recently, the research has been broadened to collect energy from environment, such as wind and human motion, under which the TENG works in relatively dry conditions, because the surface triboelectrification would be greatly decreased if not totally eliminated by the presence of water. However, water vapor and liquid water are abundant and the most obvious example is ocean waves and tides that have huge amounts of mechanical energy, which is inexhaustible and not largely dictated by daytime, season, weather and climate, in contrast to solar energy.
Until now, TENG is designed to work between solid materials and works best under dry conditions. However, tribolelectricity does exist when liquids are flowing through insulating tubes. For example, a voltage variation rising up to 300 mV is observed when deionized water flows through a 1 m-long rubber tube. Or a surface charge density of 4.5 µC m-2 is measured on each water droplet pipetted from a polytetrafluoroethylene (PTFE) tip. Therefore, herein we explore the opportunity to use water contact as one type of “material” choice for TENG. We demonstrate that the contact electrification between water and insulating polymer films can also be useful for TENG, which can derive a new application of TENG especially in liquid environments for sensing.—Lin et al.
As a prototype, the researchers made an insulated plastic tank, the lid and bottom of which contain copper foil electrodes. The inside of the lid is coated with a layer of polydimethylsiloxane (PDMS) patterned with tiny nanoscale pyramids. The tank is filled with deionized water.
When the lid is lowered so that the PDMS nanopyramids come into contact with the water, groups of atoms in the PDMS become ionized and negatively charged. A corresponding positively charged layer forms on the surface of the water. The electric charges are maintained when the PDMS layer is lifted out of the water. This produces a potential difference between the PDMS and the water.
Hydrophobic PDMS was chosen in order to minimize the amount of water clinging to the surface; the pyramid shapes allow the water to drop off readily. Periodic raising and lowering of the lid while the electrodes are connected to a rectifier and capacitor produces a direct current that can be used to light an array of 60 LEDs. In tests with salt water, the generator produced a lower output, but it could in principle operate with seawater.
The current produced decreases significantly as temperature increases, which could allow this device to be used as a temperature sensor. It also decreases when ethanol is added to the water, which suggests potential use of the system as a chemical sensor. By attaching probe molecules with specific binding partners, it may be possible to design sensors for biomolecules.
In summary, we have demonstrated a newly designed TENG based on the contact electrification between a patterned PDMS pyramid array and water. This new prototype water–TENG provided an open-circuit voltage of 52 V and a short-circuit current density of 2.45 mA m-2 with a peak power density of nearly 0.13 W m-2, which is able to drive 60 LEDs simultaneously....Tap water and deionized water with a similar ion concentration to sea water were also evaluated and showed the potential for harvesting water- related energy from the environment. Compared with traditional TENGs that are designed for the contact of solid materials, this study opens the possibility in utilizing liquid movements and extends its application scope. Furthermore, we believe the electrical output of the water–TENG could be enhanced in the future by using superhydrophobic nanostructures as the contact materials or functionalizing material surface with specific groups. This work will also inspire the development of TENG toward directly sensing metal ions and biomolecules in solution samples.—Lin et al.
Zong-Hong Lin, Gang Cheng, Long Lin, Sangmin Lee, and Zhong Lin Wang (2013) “Water-Solid Surface Contact Electrification and its Use for Harvesting Liquid Wave Energy” doi: 10.1002/anie.201307249